Devices and Methods for Electromagnetic Measurement of Axial Flow

ABSTRACT

An embodiment method for measuring fluid flow in a casing includes inserting a logging tool into the casing, wherein the logging tool having an internal axial flow channel and an electromagnetic flowmeter sensor disposed in the internal flow channel. The method also includes measuring an axial conductive fluid flow through the flow channel with the electromagnetic flowmeter sensor, while allowing bypass axial fluid flow to bypass the internal flow channel substantially unimpeded between an exterior of the logging tool and an interior wall of the casing.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to the following co-pending and commonly-assigned patent applications: U.S. patent application Ser. No. 13/561,973, entitled “Fluid Flow Measuring Device and Method,” filed Jul. 30, 2012; U.S. patent application Ser. No. 13/681,047, entitled “Apparatus and Method for Fluid Flow Measurement with Sensor Shielding,” filed Nov. 19, 2012; U.S. patent application Ser. No. 13/447,962, entitled “Rotating Fluid Measurement Device and Method,” filed Apr. 16, 2012; and International Application No. PCT/US2012/036951, entitled “Fluid Flow Measurement Sensor, Method, and Analysis,” filed on May 25, 2012, all of which applications are hereby incorporated herein by reference.

TECHNICAL FIELD

The present invention relates generally to devices and methods for flow measurement, and, in particular embodiments, to devices and methods for electromagnetic measurement of axial flow.

BACKGROUND

There are many types of sensors used for flowmeters, and various ones of these have been used or proposed to measure fluid flow in boreholes and wells. One example of a flowmeter is a mechanical spinner/impeller flowmeter, where moving fluid drives the impeller, and the rate of rotation of the impeller provides an indication of the fluid velocity. Such impellers may not properly operate at low fluid flow rates, and thus may not correctly measure slowly-moving fluid.

Another example is an electromagnetic flowmeter, which functions by inducing a voltage when a medium (such as water) moves in a magnetic field. The induced voltage is perpendicular both to the direction of the magnetic field and to the direction of the movement of the medium. When the moving medium is at least slightly conductive, an induced voltage provides an indication of the velocity of the medium. This induced voltage is directly proportional to the velocity of the moving medium.

U.S. Pat. Nos. 5,297,425 and 5,388,455 to Hamby et al., which applications are hereby incorporated herein by reference, describe the use of an electromagnetic flowmeter in water producing wells. A single flow channel with a single pair of electrodes are used with an inflatable packer filled with water pumped in from the surface to attempt to force all the flow in the well to go through the single flow channel. Alternatively, a hard collar may be used instead of a packer to prevent fluid from around the single flow channel. Generally, with such schemes there is leakage around the packer or collar, causing an inaccurate reading of the fluid flow. Also, the possibility of the tool becoming stuck in the well can be a significant problem.

SUMMARY OF THE INVENTION

An embodiment method for measuring fluid flow in a casing includes inserting a logging tool into the casing, wherein the logging tool having an internal axial flow channel and an electromagnetic flowmeter sensor disposed in the internal flow channel. The method also includes measuring an axial conductive fluid flow through the flow channel with the electromagnetic flowmeter sensor, while allowing bypass axial fluid flow to bypass the internal flow channel substantially unimpeded between an exterior of the logging tool and an interior wall of the casing.

Another embodiment method for measuring fluid flow in a casing includes inserting a logging tool into the casing, wherein the logging tool includes a tool body and an electromagnetic flowmeter sensor, wherein the electromagnetic flow sensor has a pair of electrodes disposed on an exterior of the tool body. The method also includes measuring an axial conductive fluid flow in the casing with the pair of electrodes of the electromagnetic flowmeter sensor, wherein an imaginary line between the pair of electrodes is orthogonal to the axial conductive fluid flow.

An embodiment logging tool for measuring fluid flow in a casing includes an elongated tool body having a central axis. The logging tool also includes a first non-centralized flow channel mechanically coupled to the tool body, wherein a first axis of the first flow channel is parallel to the central axis of the tool body, and wherein the first flow channel is radially offset from the central axis of the tool body, and a first electromagnetic flowmeter sensor, wherein the first electromagnetic flow sensor has a first pair of electrodes disposed inside the first flow channel, wherein a first imaginary line between the first pair of electrodes is orthogonal to the first axis of the first flow channel. The logging tool further includes a second non-centralized flow channel mechanically coupled to the tool body, wherein a second axis of the second flow channel is parallel to the central axis of the tool body, and wherein the second flow channel is radially offset from the central axis of the tool body, and a second electromagnetic flowmeter sensor, wherein the second electromagnetic flow sensor has a second pair of electrodes disposed inside the second flow channel, wherein a second imaginary line between the second pair of electrodes is orthogonal to the second axis of the second flow channel.

Another embodiment logging tool for measuring fluid flow in a casing includes an elongated tool body having a central axis, and a plurality of electromagnetic flowmeter sensors mechanically coupled to the tool body and disposed in a first plane perpendicular to the central axis of the tool body, wherein each of the electromagnetic flowmeter sensors has a flow channel with a flow channel axis parallel to the central axis of the tool body.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which:

FIG. 1 illustrates an embodiment electromagnetic flowmeter disposed in a casing;

FIGS. 2A and 2B illustrate a top view and side view, respectively, of an embodiment external electrode sensor;

FIGS. 3A, 3B and 3C illustrate several magnetic flux generator embodiments;

FIGS. 4A and 4B illustrate a top view and side view, respectively, of an embodiment having one pair of external electrodes;

FIGS. 5A and 5B illustrate a top view and side view, respectively, of an embodiment having three pairs of external electrodes;

FIGS. 6A and 6B illustrate a top view and side view, respectively, of an embodiment having eight pairs of external electrodes;

FIGS. 7A and 7B illustrate a top view and side view, respectively, of flow channels fixed on the outside of the tool body;

FIGS. 8A and 8B illustrate a top view and side view, respectively, of a logging tool with flow channels partially cut into the tool body;

FIGS. 9A and 9B illustrate a top view and side view, respectively, of an electromagnetic flow meter;

FIGS. 10A, 10B and 10C illustrate various arrangements for multiple electromagnetic flow meter sensors mounted on arms extending out from a logging tool body;

FIGS. 11A and 11B illustrate side views of a logging tool having a rotatable arm the non-deployed and deployed positions, respectively;

FIGS. 12A and 12B illustrate embodiments of a mesh approach to obtaining measurements over substantially an entire cross-sectional flow area; and

FIGS. 13A, 13B, 13C and 13D illustrate further embodiments of a mesh arrangement.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.

The co-assigned patent applications cross-referenced hereinabove generally describe the use of an electromagnetic flowmeter oriented to measure radial fluid flow into/out of a borehole wall, where the electromagnetic flowmeter is disposed adjacent the borehole wall. These applications, which are hereby incorporated herein by reference, describe detailed devices and methods for implementing electromagnetic flowmeters, and these devices and methods may be used to implement the electromagnetic flowmeters provided herein. These applications also describe systems and methods for reading, retrieving, interpreting, and analyzing data provided by electromagnetic sensors, and these systems and methods may be combined with the electromagnetic flowmeter devices and methods provided herein.

The present invention will be described with respect to preferred embodiments in a specific context, namely electromagnetic flowmeter devices and methods for measuring longitudinal axial flow in a well borehole. The invention may also be applied, however, to other applications where the detection of fluid flow is useful, such as in pipes, casings, drill shafts, tanks, and swimming pools. Embodiments may be used in vertical, deviated, and horizontal wells, and may be used in tubing, casing, slotted screens, slotted liners, and almost any well completion. Any type of conduit, wellbore, borehole, cylinder, pipe, shaft, tube, etc., is referred to herein generally as a casing.

In one embodiment, an electromagnetic flow meter device and method measure fluid flow in a casing without bypass prevention. In another embodiment, an electromagnetic flow meter device and method measure fluid flow using electrodes disposed on an exterior of a tool instead of or in addition to electrodes disposed internally in a tool, that is, in a tool flow channel. In another embodiment, an electromagnetic flow meter device and method measure fluid flow using non-centralized flow channels in the logging tool body. In another embodiment, an electromagnetic flow meter device and method measure fluid flow using multiple electromagnetic flowmeter sensors in the flow stream. The sensors may be stationary, or they may rotate around the longitudinal axis of the tool, or they may move radially in and out relative to the logging tool.

Again, an embodiment electromagnetic flow meter device and method measure fluid flow using an electromagnetic flowmeter without bypass prevention. An embodiment uses an electromagnetic flow meter only, without a packer, collar, or other flow by-pass prevention device, to measure axial flow in a wellbore. The electromagnetic flow meter can be positioned approximately centralized in the cross section of the logging tool and in the borehole, but may be positioned off-center.

The electromagnetic flowmeter is oriented to measure the axial flow of fluid in the casing. That is, both an electromagnetic field generated by the electromagnetic flowmeter, and an imaginary line between voltage-sensing electrodes in the flowmeter, are perpendicular to the axial flow of fluid in the casing, as well as to each other. The electromagnetic field and the imaginary line thus are oriented to be in the plane of the casing cross-section. Within this plane, they may be oriented in any direction, as long as they remain perpendicular to each other for the sensor to provide maximum sensitivity to axial fluid flow. It is possible for the electromagnetic field and the imaginary line to deviate from being at right angles to each other and to the axis of the casing, but the higher these deviations, the less sensitive the sensor is to the axial flow.

Thus an electromagnetic flow meter may be used in a similar manner as a conventional spinner (impeller) flowmeter, except that an electromagnetic flow meter measures only the flow of conductive fluids, such as water, and does not measure the flow of non-conductive fluids such as oil and gas. The electromagnetic flow meter optionally may have a funnel shaped entrance to facilitate the entry of fluid into its cavity/cavities containing the sensor/sensors.

The electromagnetic flow meter may include any of a number of combinations of a flow channel, a magnetic flux generator, and a pair of electrodes to detect the induced voltage that is indicative of the axial flow velocity of the conductive fluid. Such variations are described in the previously-mentioned cross-referenced patent applications.

Further, standard methods of deriving the flow rate of the conductive fluid through the entire wellbore may be used, as well as those methods described in the previously-mentioned cross-referenced patent applications.

FIG. 1 illustrates an embodiment electromagnetic flowmeter 100 for measuring the axial flow rate of a conductive fluid, such as water, in a casing 102 disposed in a borehole. The axial fluid flow 104 in the casing both passes through flowmeter 100 and bypasses flowmeter 100, as shown by bypass fluid flow 106. Flowmeter 100 functions without the use a fluid bypass prevention device, such as a packer or collar, to provide a measurement of the axial fluid flow 104. The flowmeter 100 may traverse the casing and take measurements at one or more axial locations in the casing 102. The flow rate of the axial fluid flow may be determined at various depths in the casing, and used to determine operational characteristics of the borehole. The bypass fluid flow may be allowed to flow around the logging tool substantially unimpeded, with the exception of, e.g., centralizers and stabilizers used to position and stabilize the logging tool, respectively.

Advantageously, in an embodiment, the bypass flow 106 can be accounted for with an area of flowmeter to area of internal diameter of the casing correction. Therefore, assumptions about or corrections for a packer or collar leaking do not need to be made. Packers and collars frequently leak, causing interpretation errors. Also, packers tend to become stuck in the casing, causing serious and expensive operational problems.

Additionally, for deeper wells, the running of fluid line(s), which is used to fill an expandable packer, becomes impractical. Also, running with a fixed collar around the sensor to prevent bypass flow is usually not practical in oil and gas wells due to the small diameter tubing that is normally used in portions of the wellbore.

In addition, an embodiment electromagnetic flow meter sensor has several advantages over conventional spinners. For example, there generally are no moving parts in an electromagnetic flow sensor, and moving parts, such as spinners, generally require frequent and expensive maintenance. A rotating spinner also frequently becomes jammed and stops rotating, thereby providing no useful information about flow rate. In contrast, an embodiment electromagnetic flow meter simply has an essentially open flow channel, and thus has fewer obstructions and is much more insensitive to debris in the flow stream.

Electromagnetic flow meters also have no threshold (i.e., have a zero threshold) flow rate, unlike mechanical spinners, which have thresholds typically from 4 feet per minute (FPM) to 20 FPM or more. Electromagnetic flow meters also have a linear response to velocity, whereas mechanical spinners have a linear response over higher ranges of fluid velocity, but not at low velocities where the response is very non-linear. There also is a dead zone at flow rates lower than the threshold.

Another embodiment system and method use an electromagnetic flowmeter with external electrodes. Instead of an electromagnetic flow meter with electrodes inside a flow channel in the sensor, an embodiment provides an alternative, namely a sensor that has electrodes external to the body of the sensor. This embodiment may be used in conjunction with the previous embodiment, where a bypass flow can occur inside the logging tool. Alternatively, there may be sensors disposed both internally and externally to the body of the sensor.

An embodiment with external electrodes provides a smaller, more compact device, which in some cases is a significant operational advantage, as normally one has to run the sensor through smaller diameter tubing and other restrictions before encountering the larger diameter casing where the measurements usually are taken.

Additionally, external electrodes operationally make maintenance of the sensor easier, as external electrodes can be more accessible and easier to clean, and/or replace, if needed.

FIGS. 2A and 2B illustrate a top view and side view, respectively, of an embodiment logging tool 200 having an electromagnetic flowmeter with external electrodes 202. A magnetic flux generator 204 has magnetic core 206 with magnetic pole faces 208. Magnetic flux generator 204 generates magnetic flux 206 between the electrode pair 202, such that fluid flow 210 generates an induced voltage 212 between electrodes 202. The electromagnetic flowmeter thus is configured to measure axial fluid flow in a casing where the fluid is flowing externally to the logging tool along its long axis.

FIGS. 3A, 3B and 3C illustrate several embodiments for magnetic flux generator 204 used for an electromagnetic flow meter on logging tool 200. There are many variations of these devices and methods, all of which may be used to provide magnetic flux for the electromagnetic flow meter. For example, any of the variations described in the previously-mentioned cross-referenced patent applications may be used as a magnetic flux generator.

In FIG. 3A, magnetic flux generator 204A has magnetic core 306A with core pole faces 308A. Current source 310A provides an electrical current to wire coil 312A that is wrapped around the magnetic core, thus increasing the strength of the magnetic flux. The current source may provide direct or alternating current, as disclosed in the previously-mentioned cross-referenced patent applications.

In FIG. 3B, magnetic flux generator 204B has magnetic core 320B with core pole faces 322B. Current source 324B provides an electrical current to wire coil 326B that is wrapped around the magnetic core, thus increasing the strength of the magnetic flux. In this embodiment, a larger coil is used to further increase the strength of the magnetic flux. The current source may provide direct or alternating current, as disclosed in the previously-mentioned cross-referenced patent applications.

In FIG. 3C, magnetic flux generator 204C has magnetic core 330B with core pole faces 322B. In this embodiment the magnetic flux generator uses a permanent magnet 334C to generate the magnetic flux for the electromagnetic flow meter.

FIGS. 4A, 4B, 5A, 5B, 6A and 6B illustrate several different embodiment logging tools having external electrodes. In various embodiments, one pair of electrodes can be used, or multiple pairs of electrodes can be used, including enough electrode pairs to cover the entire outside circumference or perimeter of the logging tool. If two or more electrode pairs are used, the electrode pairs can be disposed at different axial levels along the length of the logging tool, or at the same level (branched poles), or some combination of the two.

FIGS. 4A and 4B illustrate a top view and side view, respectively, of an embodiment having one pair of electrodes. Logging tool 400 has electrode pair 402 disposed on an exterior of the tool perpendicular to a magnetic flux generated by pole faces 404 of a magnetic flux generator. An induced voltage across the electrodes caused by fluid flow perpendicular to both the electrodes and the magnetic flux may be measured to determine the fluid flow rate.

FIGS. 5A and 5B illustrate a top view and side view, respectively, of an embodiment having three pairs of electrodes. Logging tool 500 has electrode pairs 502 disposed evenly around the circumference of an exterior of the tool, such as with 120 degree spacing. Alternatively, the electrode pairs may be disposed with irregular spacing on the exterior of the tool. Each of the pairs of electrodes is offset from the others along the length of the logging tool sensor, although two or all of them may be disposed at a same axial level of the sensor. Each of the pairs of electrodes 502 is disposed perpendicular to a magnetic flux generated by respective pole faces 504 of a magnetic flux generator. A single magnetic flux generator may be used to generate the magnetic fluxes, or separate magnetic flux generators may be used. An induced voltage across one or more of the pairs of electrodes caused by fluid flow perpendicular to both the electrodes and the magnetic flux may be measured to determine the fluid flow rate.

FIGS. 6A and 6B illustrate a top view and side view, respectively, of an embodiment having multiple pairs of electrodes surrounding the external circumference of logging tool. Logging tool 600 has eight electrode pairs 602 disposed evenly around the circumference of an exterior of the tool, such as with 45 degree spacing. Alternatively, the electrode pairs may be disposed with irregular spacing on the exterior of the tool, or may be disposed around the circumference in a non-sequential manner. Each of the pairs of electrodes is offset from the others along the length of the logging tool sensor, although two or more of them may be disposed at a same axial level of the sensor. Each of the pairs of electrodes 602 is disposed perpendicular to a magnetic flux generated by respective pole faces 604 of a magnetic flux generator. A single magnetic flux generator may be used to generate the magnetic fluxes, or separate magnetic flux generators may be used. An induced voltage across one or more of the pairs of electrodes caused by fluid flow perpendicular to both the electrodes and the magnetic flux may be measured to determine the fluid flow rate. Multiple pairs of electrodes may be used to determine differing flow rates in different areas of a cross-section of a casing.

Another embodiment system and method use an electromagnetic flowmeter with non-centralized flow channels in the logging tool body. There may be any number of such flow channels, including any specific number between two and eight, or more, disposed on the tool body. One or more of the flow channels may be located fully or partially inside the body of the logging tool. The electrode pairs can have various arrangements, such as circumferential, radial, etc. This embodiment may be used in conjunction with the previous embodiments, where there may be a non-centralized flow channel in combination with a centralized flow channel. Alternatively, there may be non-centralized flow channel in combination with an external electrode pair without a dedicated flow channel.

FIGS. 7A and 7B illustrate a top view and side view, respectively, of an embodiment having two or more flow channels 704 fixed in tool body 702 of logging tool 700. Each of the flow channels includes at least the electrodes of an electromagnetic flow meter within it. The flow channels may have variously shaped cross sections, such as rounded, rectangular, etc., as described in the previously-mentioned cross-referenced patent applications. The flow channels can be placed in a scalloped-out section 706 of the tool string to allow the wellbore flow to reach the sensors in the flow channels. Alternatively, the flow channels can be placed at the bottom end of a full diameter logging tool string with a scalloped-out section above the sensors, or at the top end with a scalloped-out section below the sensors.

FIGS. 8A and 8B illustrate a top view and side view, respectively, of an alternative embodiment having one, two or more flow channels 804 that are partially cut into the tool body 802 of logging tool 800. Each of the flow channels includes at least the electrodes of an electromagnetic flow meter within it. The flow channels may have variously shaped cross sections, such as rounded, rectangular, etc., as described in the previously-mentioned cross-referenced patent applications, except that the flow channels have open sides. The flow channels can be placed in any section of the tool string that allows the wellbore flow to reach the sensors in the flow channels.

Another embodiment system and method use one or more electromagnetic flowmeter sensors external to the body of the logging tool. The sensors can be disposed at various locations in the cross-section of the flow stream in a wellbore, and can be disposed in a single plane or in multiple planes. In various embodiments, a variety of locations in the flow stream and a variety of sensor movements can be used. These embodiments may be used in conjunction with the previous embodiments disclosed herein. Further, as with other embodiments disclosed herein, the electromagnetic sensors, arm configurations, arm radial and circumferential movements, magnetic flux generators, electrical current shields, resistor and electrode networks, electronic control and measurement circuits, measurement procedures and analyses, etc., disclosed in the previously-mentioned cross-referenced patent applications may be used in conjunction with these embodiments.

FIGS. 9A and 9B illustrate a top view and side view, respectively, of an embodiment small electromagnetic flow meter 900 that can be placed away from the body of the logging tool on which the flow meter is mounted. Magnetic flux generator 902, such as a permanent magnet or a coil around an iron core 904, generates a magnetic flux. The magnetic flux passes between the two pole faces 906. Movement of conductive fluid 908 induces a measurable voltage difference between the two electrodes 910. Leads from the electrodes 910 provide a measurement related to the velocity of the conductive fluid. These sensors can be quite small, providing a minimal obstacle to the flow of fluid in a well bore. The actual sensing volume can be, for example, 1 mm by 3 mm by ½ mm or even smaller. This sensing volume can be adjusted as needed by modifying the physical dimensions of the pole faces and the separation of the electrodes, in accordance with a particular application. Shield 912, which is insulating but also may be conducting, is optional but often useful to prevent any stray circulating electrical currents from adversely affecting the measurement, especially when more than one pair of electrodes are used.

FIGS. 10A, 10B and 10C illustrate various arrangements for multiple electromagnetic flow meter sensors mounted on arms extending out from the body of a logging tool. The flow in a wellbore is often not uniform and not uncommonly can be dramatically different in one part of a given cross section of a wellbore than in another part of the cross section. For example, the flow can be in the opposite direction on one side of the wellbore cross section from that of the other side. Likewise the axial flow can be different at different axial positions in the wellbore. Because the axial flow in the wellbore can be complex and varied even in relatively close regions, the multiple sensors generally will provide a more accurate indication of this complex fluid flow than a single sensor.

A different number of sensors, such as any specific number between two and 12, or more, can be implemented in various embodiments. The radial locations of the sensors away from the tool body can be varied within an embodiment or between different embodiments. Also, the axial locations of the sensors along the logging tool body can be varied within an embodiment or between different embodiments. The electromagnetic sensors can be self-contained sensors, with the arm for each providing mechanical and positioning support, as well as a conduit or support for the wires carrying, e.g., data, control and power signals. Alternatively, each arm can be implemented as part of the core of its respective sensor. Further, there may be more than one sensor mounted on each arm, either at the end of the arm, or at multiple locations along the arm.

FIG. 10A illustrates six electromagnetic flowmeter sensors 1004A extended out from the body 1002A of logging tool 1000A on arms 1006A. As shown in FIG. 10A, sensors 1004A can be placed in multiple radial positions (distances radially away from the tool body) and at multiple circumferential positions (angles around the tool body) to provide readings of the flow at multiple locations in a wellbore. As further shown, the sensors can be placed in multiple axial positions along the length of the tool body.

FIG. 10B illustrates three electromagnetic flowmeter sensors 1004B extended out from the body 1002B of logging tool 1000B on arms 1006B. Electromagnetic flowmeter sensors 1004B can be moved radially in and out during logging to more fully sample the flow at various radial parts of the wellbore.

FIG. 10C illustrates three electromagnetic flowmeter sensors 1004C extended out from the body 1002C of logging tool 1000C on arms 1006C. Electromagnetic flowmeter sensors 1004C can be moved circumferentially around the wellbore during logging to more fully sample the flow at various angular parts of the wellbore. In another embodiment, both the radial movement of FIG. 10B and the circumferential movement of FIG. 10C can be combined and used at the same time.

FIGS. 11A and 11B illustrate side views of a logging tool 1100 having a string of flow channels 1104 in the non-deployed and deployed positions, respectively. Each flow channel 1104 contains an electromagnetic flow meter, and is mounted on rotatable arm 1106. The logging tool 1100 can be run into a wellbore with the arm 1106 in an orientation approximately aligned with the axis of the tool body 1102, and then, once below the tubing and other restrictions in the wellbore, arm 1106 can be rotated into an orientation that is approximately perpendicular to the axis of the tool body 1102, in order to take measurements of axial fluid flow 1108. In one embodiment the deployed arm has a fixed circumferential orientation with respect to the logging tool. In another embodiment, the arm 1106 with the string of flow channels 1104 can be rotated about the axis of the logging tool to sample more regions of the wellbore cross section.

FIGS. 12A and 12B illustrate examples of various embodiments of a mesh approach to obtaining measurements over substantially the entire cross-sectional flow area within the wellbore. Because flow frequently travels with substantially different rates at different portions of the borehole, an approach sampling a large portion of the borehole would increase the accuracy of the flow measurement.

With a mesh arrangement, magnetic flux is generated over substantially the entire flow area within the wellbore, and measurements of the flow-induced voltages are made at a multitude of locations within the flow stream using a multitude of electrode pairs or the equivalent. The magnetic flux orientation and the electrode orientation are substantially perpendicular to each other, and they are both substantially perpendicular to the axial flow. This multitude of flow-induced voltages then can be used to determine the overall flow rate of the entire flow within the wellbore. When measuring the induced voltage due to fluid flow, several approaches can be taken.

FIG. 12A illustrates an embodiment of the mesh arrangement approach using multiple electrode pairs 1202A to measure fluid flow in wellbore 1200A. Magnetic flux generator 1204A (on or in the logging tool) is disposed in the middle of the wellbore, and generates magnetic flux 1206A perpendicular to the electrode pairs 1202A. One approach for measuring the induced voltage is to measure it across each of the multitude of electrode pairs. Another approach is to electrically sum the multiple induced voltages across some or all electrode pairs using a resistor chain. The summing resistors can be part of the mesh or located elsewhere.

FIG. 12B illustrates another mesh arrangement approach using multiple long electrode pairs 1202B to measure fluid flow in wellbore 1200B. Similar to FIG. 12A, magnetic flux generator 1204B (on or in logging tool) is disposed in the middle of the wellbore, and generates magnetic flux 1206B. In this embodiment, the multiple individual electrode pairs on each side of a given flux line are replaced with two long electrodes 1202B. The voltage between the two long electrodes 1202B on either side of a magnetic flux line 1206B is measured. This can be repeated by the use of multiple long electrodes pairs to cover substantially the entire flow area.

FIGS. 13A, 13B, 13C and 13D illustrate further embodiments of a mesh arrangement. In these embodiments, limbs of high magnetic permeability material extend outward from the magnetic flux generator or tool body. The magnetic flux generated by the magnetic flux generator transverses from one to another of the limbs. While not all flux lines are shown, the pattern of the flux lines is modified from the previous embodiments by the limbs. Electrode pairs in a roughly mesh like pattern, such as the partial patterns shown in the figures, can be used to measure the induced voltages. As before, the induced voltages can be measured individually at individual electrode pairs, or summed by a resistor chain network. Further, the induced voltages can be measured separately at individual long electrode pairs, or summed by a resistor chain network.

FIG. 13A illustrates an embodiment mesh arrangement 1300A. Magnetic flux generator 1302A (on or in the logging tool) is disposed in the middle of the wellbore, and generates magnetic flux 1304A, which is modified by high magnetic permeability limbs 1306A. Electrode pairs 1308A (only some of which are shown) are used to measure the flow rate of fluid flowing axially past the electrode pairs. One approach for measuring the induced voltage is to measure it across each of the multitude of electrode pairs. Another approach is to electrically sum the multiple induced voltages across some or all electrode pairs using a resistor chain. The summing resistors can be part of the mesh or located elsewhere.

FIG. 13B illustrates another embodiment mesh arrangement 1300B. Magnetic flux generator 1302B is disposed in the middle of the wellbore, and generates magnetic flux 1304B, which is modified by high magnetic permeability limbs 1306B. Long segmented electrode pairs 1308B (only one pair about one flux is shown) are used to measure the flow rate of fluid flowing axially past the electrode pairs. Alternatively, multiple segments may be used for one flux line. One approach for measuring the induced voltage is to measure it across each of the multitude of long segmented electrode pairs. Another approach is to electrically sum the multiple induced voltages across some or all of the long segmented electrode pairs using a resistor chain. The summing resistors can be part of the mesh or located elsewhere.

FIG. 13C illustrates another embodiment mesh arrangement 1300C. Magnetic flux generator 1302C is disposed in the middle of the wellbore, and generates magnetic flux 1304C, which is modified by high magnetic permeability limbs 1306C. In this embodiment, teeth 1310C are placed at various locations along the magnetic limbs to increase the magnetic flux density near the location of teeth 1310C. Electrode pairs 1308C (only some of which are shown) are used to measure the flow rate of fluid flowing axially past the electrode pairs. One approach for measuring the induced voltage is to measure it across each of the multitude of electrode pairs. Another approach is to electrically sum the multiple induced voltages across some or all electrode pairs using a resistor chain. The summing resistors can be part of the mesh or located elsewhere.

FIG. 13D illustrates another embodiment mesh arrangement 1300D. Magnetic flux generator 1302D is disposed in the middle of the wellbore, and generates magnetic flux 1304D, which is modified by high magnetic permeability limbs 1306D. In this embodiment, teeth 1310C are placed at various locations along the magnetic limbs to increase the magnetic flux density near the location of teeth 1310C. Long segmented electrode pairs 1308D (only one pair about one flux is shown) are used to measure the flow rate of fluid flowing axially past the electrode pairs. Alternatively, multiple segments may be used for one flux line. One approach for measuring the induced voltage is to measure it across each of the multitude of long segmented electrode pairs. Another approach is to electrically sum the multiple induced voltages across some or all of the long segmented electrode pairs using a resistor chain. The summing resistors can be part of the mesh or located elsewhere.

While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments. 

What is claimed is:
 1. A method for measuring fluid flow in a casing, the method comprising: inserting a logging tool into the casing, the logging tool having an internal axial flow channel and an electromagnetic flowmeter sensor disposed in the internal flow channel; and measuring an axial conductive fluid flow through the flow channel with the electromagnetic flowmeter sensor, while allowing bypass axial fluid flow to bypass the internal flow channel substantially unimpeded between an exterior of the logging tool and an interior wall of the casing.
 2. The method of claim 1, further comprising generating a magnetic flux inside the flow channel orthogonal to the axial fluid flow through the flow channel.
 3. The method of claim 2, wherein the electromagnetic flowmeter sensor comprises a pair of electrodes disposed in the flow channel, wherein an imaginary line between the pair of electrodes is orthogonal to the magnetic flux and to the axial fluid flow through the flow channel, and wherein the measuring comprises measuring a voltage difference between the pair of electrodes, wherein the voltage difference is proportional to a velocity of the axial conductive fluid flow.
 4. The method of claim 1, further comprising repeating the measuring at different depths in the casing.
 5. The method of claim 1, further comprising centering the flow channel in the casing prior to the measuring.
 6. A method for measuring fluid flow in a casing, the method comprising: inserting a logging tool into the casing, the logging tool comprising a tool body and an electromagnetic flowmeter sensor, wherein the electromagnetic flow sensor has a pair of electrodes disposed on an exterior of the tool body; and measuring an axial conductive fluid flow in the casing with the pair of electrodes of the electromagnetic flowmeter sensor, wherein an imaginary line between the pair of electrodes is orthogonal to the axial conductive fluid flow.
 7. The method of claim 6, further comprising generating a magnetic flux orthogonal to the axial fluid flow and to the imaginary line between the pair of electrodes.
 8. The method of claim 6, wherein the measuring comprises measuring a voltage difference between the pair of electrodes, wherein the voltage difference is proportional to a velocity of the axial conductive fluid flow.
 9. The method of claim 6, wherein the logging tool further comprises a plurality of electromagnetic flowmeter sensors each having a respective pair of electrodes disposed on the exterior of the tool body, and wherein the measuring further comprises, for each respective pair of electrodes, measuring a voltage difference between the respective pair of electrodes, wherein the voltage difference is proportional to a velocity of the axial conductive fluid flow between the respective pair of electrodes.
 10. The method of claim 9, wherein the measuring further comprises measuring the axial conductive fluid flow at multiple angular locations around a perimeter of the tool body.
 11. A logging tool for measuring fluid flow in a casing, the logging tool comprising: an elongated tool body having a central axis; a first non-centralized flow channel mechanically coupled to the tool body, wherein a first axis of the first flow channel is parallel to the central axis of the tool body, and wherein the first flow channel is radially offset from the central axis of the tool body; a first electromagnetic flowmeter sensor, wherein the first electromagnetic flow sensor has a first pair of electrodes disposed inside the first flow channel, wherein a first imaginary line between the first pair of electrodes is orthogonal to the first axis of the first flow channel; a second non-centralized flow channel mechanically coupled to the tool body, wherein a second axis of the second flow channel is parallel to the central axis of the tool body, and wherein the second flow channel is radially offset from the central axis of the tool body; and a second electromagnetic flowmeter sensor, wherein the second electromagnetic flow sensor has a second pair of electrodes disposed inside the second flow channel, wherein a second imaginary line between the second pair of electrodes is orthogonal to the second axis of the second flow channel.
 12. The logging tool of claim 11, wherein: the first electromagnetic flowmeter sensor comprises a first magnetic flux generator configured to generate a first magnetic flux orthogonal both to the first imaginary line between the first pair of electrodes and to the first axis of the first flow channel; and the second electromagnetic flowmeter sensor comprises a second magnetic flux generator configured to generate a second magnetic flux orthogonal both to the second imaginary line between the second pair of electrodes and to the second axis of the second flow channel.
 13. The logging tool of claim 11, wherein the tool body has a scalloped-out section, and wherein the first and second flow channels are disposed on the tool body in the scalloped-out section.
 14. The logging tool of claim 11, wherein each of the first and second flow channels is partially embedded in a side of the tool body, and has an open side radially away from the tool body.
 15. The logging tool of claim 11, wherein there are multiple pairs of electrodes disposed in each of the flow channels.
 16. A logging tool for measuring fluid flow in a casing, the logging tool comprising: an elongated tool body having a central axis; a plurality of electromagnetic flowmeter sensors mechanically coupled to the tool body and disposed in a first plane perpendicular to the central axis of the tool body, wherein each of the electromagnetic flowmeter sensors has a flow channel with a flow channel axis parallel to the central axis of the tool body.
 17. The logging tool of claim 16, further comprising additional electromagnetic flowmeter sensors mechanically coupled to the tool body, and disposed in a second plane perpendicular to the central axis of the tool body different from the first plane.
 18. The logging tool of claim 16, wherein the electromagnetic flowmeter sensors are configured to move radially inward toward and outward from the tool body, and/or rotate about the central axis of the tool body.
 19. The logging tool of claim 16, wherein the electromagnetic flowmeter sensors are mounted on a plurality of arms mechanically coupled to the tool body.
 20. The logging tool of claim 19, wherein more than one of the electromagnetic flowmeter sensors is mounted on one of the arms. 